Ball screws are classified into several types depending on the ball circulation systems as follows: the end cap type, the return tube type, the circulation piece type, and the guide plate type.
Here, an end cap type ball screw will be described with reference to FIG. 5 showing an axial longitudinal sectional view thereof.
The end cap type ball screw 30 comprises a screw shaft 40 including a helical groove 42 in the outer peripheral surface thereof, a ball nut 50 including in the inner peripheral surface thereof a helical groove 52 opposed to the helical groove 42 of the screw shaft 40 and including therein a ball return passage 54 consisting of a through hole extending axially.
The ball nut 50 has a pair of end caps 60, 60 each of which is mounted to a respective axial end, and thereby the helical groove 42 of the screw thread 40, the helical groove 52 of the ball nut 50, and the ball return passage 54 are connected to define a ball circulation passage 58.
With the ball screw 30, balls 10 disposed within the ball circulation passage 58 collide with and rub against each other when they move rotationally, therefore an undesirable noise (generally, represented by "sound pressure levels") is produced during use of the ball screw 30, and balls 10 wear out because of the friction therebetween.
Further, since the balls 10 are jostling with each other while in use, the balls 10 are hard to roll to cause a locking phenomenon, which may result in clogging of the balls, especially when the ball screw 30 is used while being subjected to preload.
As a consequence, there is proposed a ball screw wherein a so-called "spacer ball" is interposed between bearing balls in order to prevent the ball clogging.
As the spacer ball is designed to have a diameter slightly smaller than that of the bearing ball, the spacer ball is not required to bear the load. Thus, the ball screw is adapted to rotationally move while rolling the bearing balls and the spacer balls in the reverse direction to each other, thereby adjacent balls are prevented from rolling in the same direction and from jostling with each other.
As is disclosed in Japanese Unexamined Patent Publication No. 169746/1998, there is proposed a ball screw employing a so-called "ball chain" in which each of balls is incorporated into a connecting member so as to be held therein, and surrounded by a belt-like member so as not to fall from the connecting member.
A ball screw employing such a ball chain can avoid contact between balls within the circulation passage, reduce the wear of the balls, and prevent the occurrence of noise and ball clogging.
However, the ball screw using the aforementioned spacer balls includes therein bearing balls and spacer balls which are arranged in an alternate manner, and adjacent balls remain in contact. As a result, this ball screw involves the problem that the early wear, the occurrence of noise, and the expansion and contraction caused by the frictional heat are hard to control.
In addition, even for the ball screw employing such a spacer ball, it is necessary, in order to prevent the balls from clogging, to reduce the number of balls by two to four from a predetermined number of balls to be installed. Consequently, such a ball screw is accompanied with the problems that the bearing capacity thereof decreases corresponding to the number of the reduced load-bearing balls and that the bearing capacity also decreases for the above-mentioned reason due to using the spacer balls.
Further, when a ball screw using spacer balls is preloaded, variation of the driving torque (which is referred to as "dynamic preload torque") of the ball screw increases and therefore smooth driving cannot be obtained.
Still further, when the ball screw is brought into repetitive operations with minute feeds, the ball clogging phenomenon occurs due to the mutual friction between balls even if the spacer balls are used.
Besides, when a ball chain is used, a concave channel must be formed in an interior wall for receiving the connecting member of the ball chain, and therefore an additional manufacturing cost is required.
Additionally, for a current mass-produced ball chain, the overall length thereof is predetermined according to its model number and this length cannot be changed appropriately, and therefore it is required to form a ball circulation passage with high dimensional accuracy so as to meet the predetermined length of the ball chain.
In fact, however, it is difficult to make uniform the overall lengths of the ball circulation passage and the ball chain. If the overall length of the ball chain is shorter than that of the ball circulation passage, then a gap is formed between the passage and the starting and terminal ends of the ball chain. On the other hand, if the overall length of the ball chain is longer than that of the ball circulation passage, the problem is caused that the ball chain cannot be installed within the ball circulation passage.